Various flow meters have been developed for the purposes of detecting and measuring water flow through and between boreholes to thereby determine the hydrology in surrounding geologic formations. However, such devices have been found to be generally unsatisfactory for measuring slow to medium fluid flow rates in low to medium viscosity fluids.
An example of a flowmeter in common use is the "spinner" flowmeter (reported by Keys, W. S., and MacCary, L. M., in 1971 in "Application of borehole geophysics to water resources investigations," U.S. Investigations, Book 2, Chapter E1, p. 109). However, the spinner flowmeter has a minimum flow velocity sensitivity of about 2 to 4 feet/minute (1 to 2 mm/sec) in wells of 4-inch or larger diameter and as high as 10 feet/minute in 2-inch diameter wells and, thus, the spinner flowmeter is not capable of measuring slow fluid flows.
Other examples of flowmeters include gas anemometers and other continuous heat-transfer devices. See, for example, Chapman, H. T., and Robinson, A. E., 1962, "A thermal flowmeter for measuring velocity of flow in a swell": U.S. Geological Survey Water-Supply Papter 1544-E, 12 p.; and Morrow, T. B., and Kline, S. J., 1971, "The evaluation and use of hot-wire and hot-film aneomometers in liquids": Standord, Calif., Stanford University, Department of Mechanical Engineering, Thermosciences Division, Report MD-25, 187 p. Although some such devices are sensitive to slow fluid flow, none can be relied upon as an accurate quantitative flowmeter under normal borehole conditions. In particular, such devices must be recalibrated frequently if the sensing surface of the device becomes coated by a substance such as mud, lint, or a mineral deposit.
Further examples of flow measuring techniques include fluid-temperature and fluid-resistivity or fluid-conductivity logs Sorey, M. L., 1971, "Measurement of vertical ground-water velocity from temperature profiles in wells": Water Resources Research, v. 7, no. 4, p. 963-970. Such techniques are useful in locating the entrance and exit of contrasting fluids in a borehole but provide little quantitative information relating to the velocity or volume of flow.
Another class flowmeters are the "tag-trace" flowmeters wherein a detectable "tagging" solution or gas is deposited in the fluid flow to be measured. The fluid flow rate is then determined by monitoring the movement of the solution or gas carried by the fluid. Tracer sensing is accomplished by means, dependent upon the nature of the "tagging" solution, such as fluid resistivity detectors, radio-active radiation detectors, florescence detectors, etc.
Of "tag-trace" flowmeters, the most commonly used are radioactive-tracer/gamma-ray detector techniques described by Bird, J. R. and Dempsey, J. C., 1955, "The use of radioactive tracer surveys in water-injection wells": Lexington, Ky. Geological Survey Special Publication 8, 10 p.; and Edwards, J. M., and Holter, E. L., 1962, "Applications of a subsurface solid-state isotope injector to nuclear-tracer survey methods": Journal of Petroleum Technology, v. 14, no. 2, p. 121-124; and brine-tracer/fluid-resistivity detector techniques described by Patten, E. R., and Bennett, G. D., 1962, "Methods of flow measuement in well bores": U.S. Geological Survey Water-Supply Paper 1544-C, 28 p. 1971; and the Keys and MacCary reference mentioned above. See, as examples, U.S. Pat. No. 4,507,552 (Roesner et al) which describes a system employing a radioactive tracer element and U.S. Pat. No. 4,805,450 (Bennett et al) which describes a system employing a tracer gas. However, such techniques are slow and are therefore expensive to perform. Also, a difference in density between the "tagging" fluid and the borehole fluid causes an uncertainty in the measurement of slow velocity flows. This uncertainty frequently exceeds the actual fluid velocity thus rendering the results completely unreliable.
A thermal-pulse borehole flowmeter was developed by "Dudgeon, C. R., Green, J. J., and Smedmore, W. J., 1975, "Heat-pulse flowmeter for boreholes": Medmenham, Marlow, Bucks, England, Water Research Centre Technical Report TR-4, 69 p; (1975) which used a tag-trace technique with a heated "tagging" solution better adapted to measuring a slow flow velocity. However, it was found that this device is not capable of withstanding the high pressure associated with deep boreholes. Further, resistance variations, occurring in a connecting cable used to connect the probe of the flowmeter to a surface station, adversely effects the measurements received by the ground station. Stray electrical currents, commonly occurring in the geologic strata surrounding a borehole, also adversely effect the measurements received by the surface station. Other disadvantages include the necessity of using a 6-conductor cable to connect the flowmeter probe to the surface station and the necessity of using a hydraulically powered centering device including a hydraulic line connecting the probe to the surface.